Chipless RFID System Research Articles

Implementation of a Low-Cost Chipless RFID System With Paper-Based Substrates Printed Tags for Traceability Applications in the Packaging Sector

Increasing competitiveness as well as levels of sustainability are some of the main challenges that the packaging sector is facing. The introduction of information and communication technologies provides new functionalities, such as traceability within logistic processes. In this context, RFID technologies can play a key role but can also pose hurdles, owing to increased cost, as well as to difficulties in relation with recycling and complying with product circularity. In this sense chipless RFID opens new possibilities, by avoiding the use of information coded chips, reducing costs in material and placement, as well by improving recyclability. In this work, a frequency based chipless RFID solution is presented, in which 2D geometrical patterns printed with conductive ink, resembling transmitive or reflective metasurfaces, are designed and integrated within paper-based substrates. An in-house reader system is implemented based on conventional Software Defined Radio platforms in the 0-6GHz and in the 0-12GHz range, in which received signal post-processing is performed to increase reader sensitivity, without the need of using Vector Network Analyzer equipment. Moreover, in order to reduce material waste, an automatic fault detection system based on artificial vision coupled to a robot arm is designed and tested. Different prototypes in paper substrates as well as in reference printed circuit boards have been fabricated and measured, showing good agreement. The proposed solution provides a feasible chipless RFID system with a portable low-cost reader which can be embedded in paper/cardboard materials in packaging applications.

Electromagnetic Modeling Strategy Supporting the Fabrication of Inkjet-Printed Chipless RFID Sensors

Inkjet printing fabrication techniques can be used for the manufacturing process of chipless radio-frequency identification RFID sensors. With respect to traditional silicon-based fabrication, flexible electronics induce the possibility to experiment new substrates and conductive materials. With the development of high-conductive nano-inks the low sintering temperature required to process them results compatible with a large variety of substrates. However, the requirements for the conductivity of the metallized printed patterns are particularly stringent for applications where the working frequency is high, like in the case of common chipless RFID tags working in the frequency range 3-10GHz. In chipless RFID systems, the higher the conductivity of the conductive parts and the better the signal response of the tag. However, in printed chipless RFIDs the conductivity can only give a partial indication of the goodness of the tag. The tag response is dependent on the sheet resistance, which is not only related to the conductivity of the ink but also to the thickness of the printed pattern. Therefore, if a sufficiently high-conductive nano-ink is selected, the signal efficiency of the printed tag can be further improved by making an evaluation based on the thickness of the patterns. The objective of this study is to propose a strategy for evaluating the signal response efficiency in inkjet-printed chipless tags by means of a first quantification of the average thickness of the printed layer, which is used to feed a finite element method simulation model to give the manufacturer an idea of the optimal number of layers to be printed in order to have a strong signal response.

Model-Based VF Technique for Parasitic Reflections Reduction of Frequency-Domain Chipless RFID Systems

In a frequency-domain chipless RFID system, the backscattered signal from a tag is directly obtained in the frequency domain over limited bandwidth. However, to reduce the parasitic reflections, it is crucial to obtain the backscattered signal in the time domain. This letter proposes a precise quasi-analytic method, known as the model-based vector fitting (VF) technique, to provide the closed-form equation of the backscattered response in the time domain. The proposed method successfully recovers a tag’s identification (ID) among parasitic reflections using a single tag measurement without requiring prior knowledge about the tag.

A Pulse Distortion Approach for Decoding Frequency-Coded Tags in Multicarrier Chipless RFID Systems

We propose a new approach based on the multicarrier system, in which multiple narrowband pulses emit toward a chipless RFID tag in the UWB, to decode the tag in the time domain. The multicarrier system is known to be able to reduce complexity and interference, and also increase spectrum efficiency and flexibility. In this study, we not only experimentally investigate the backscattered signal of a frequency-coded chipless RFID tag interrogated by narrowband pulses, but we have also derived mathematical equations to describe the tag responses. We have found that the shape of the reflected response from the tag is determined by the emitted signal multiplied by a distortion factor. The distortion amount depends on transmitted signal bandwidth, tag quality factor, and resonance frequency. We further use the distorted tag response as a tag signature in the time domain for decoding purposes, eliminating the signal transformation to the frequency spectrum, which is one of the most time-consuming post-processing algorithms. To further demonstrate the performance of the proposed approach in a more realistic scenario, we have developed a customized multicarrier transceiver to decode a four-bit cross-polarized tag in the time domain. We have demonstrated that the proposed analytical approach can also enhance tag detection in a realistic environment with multiple parasitic reflections.

A Review of Chipless RFID Measurement Methods, Response Detection Approaches, and Decoding Techniques

Chipless RFID systems can be considered as a special case of passive RFID systems, where the tag contains no power source and no electronics. Instead, the tag’s information is stored in its structure and accessed through its electromagnetic (EM) scattering response. However, robust response detection, which is primarily a function of the measurement method, measurement equipment, and processing method used, is still a major challenge in the chipless RFID field. The consequences of not properly capturing a tag response include, incorrectly assigning an ID or incorrectly reporting a sensing parameter. Due to the criticality of these challenges, this review seeks to provide an overview of the current measurement methods, equipment architectures, and processing methods as they relate to chipless RFID tag response detection and decoding. Since chipless RFID started gaining popularity around 2005, the developments in this area have been focused on three major categories: time-domain, frequency-domain, and spatial-domain systems. Frequency-domain systems have emerged as the most popular among these three categories, and thus, this review focuses on techniques used for these systems.

Towards an Efficient Chipless RFID System for Modern Applications in IoT Networks

In this article, we present the design and validation of an efficient chipless RFID system. A multi-resonator chipless tag is designed and tested for high bit coding capacity. A high gain, ultra-wideband step-shape rectangular patch (USRP) antenna is proposed to validate the specificity of the tag in terms of its operation. The devised antenna is evaluated for various performance parameters, which recommend its suitability for testing and validation of high-capacity tags that can be deployed in modern applications, particularly in the Internet of Things (IoT) networks. A measurement setup is established to achieve performance validation of the tag over a significant range of 40 cm. There is close agreement between the measured and simulated results, which suggests that the proposed antenna system can be adopted in a similar measurement setup to test and validate the performance of any chipless RFID tag operating in the same bandwidth meant for IoT networks.

Robust Chipless RFID Detection Using Complex Natural Frequency Along With the k-Nearest Neighbor Algorithm

This paper proposes a detection technique for the chipless RFID system. In the proposed technique, the experiments for measuring the scattering responses of all possible tags were conducted first in a free-space anechoic chamber in order to avoid the effect of unwanted signals due to the environment. The responses of all possible tags were exploited in order to extract poles, including natural frequencies and damping factors, by using the short-time matrix pencil method. All extracted poles successively chosen from the late-time portions were exploited to create the decision boundary by using the k- nearest neighbor algorithm. In order to validate the robustness of the proposed detection technique, experiments with the chipless RFID system with tags attached to containers, i.e. parcel and plastic boxes, were conducted. Poles extracted from the response of the tag attached to a container were fed to the decision boundary for ID detection. The poles that had fallen into the region labeled as logic “1” were detected as logic “1,” and vice versa . The experiment results showed that the attachment of the tags to the containers caused a change in the poles. The conventional technique using only natural frequencies has exhibited poor performance regarding tag ID detection. On the other hand, the proposed detection technique achieved a 100% detection rate, although the extracted poles used to detect the bit logic were disturbed by the container. The experimental results confirm the superiority of the proposed system over conventional chipless RFID detection. Moreover, the proposed technique can be considered a robust detection technique.

Radar Cross Section of Chipless RFID Tags and BER Performance

The performance of different chipless RFID tag topologies are analysed in terms of Radar Cross Section (RCS) and Bit Error Rate (BER). It is shown that the BER is mainly determined by the tag Radar Cross Section (RCS) once that a standard reading scenario is considered and a fixed size of the tag is chosen. It is shown that the arrangement of the resonators in the chipless tag plays a crucial role in determining the cross-polar RCS of the tag. The RCS of the tag is computed theoretically by using array theory where each resonator is treated as a separate scatterer completely characterized by a specific reflection coefficient. Several resonators arrangements (periodic and non-periodic) are compared, keeping the physical area of the tag fixed. Theoretical and experimental analysis demonstrate that the periodic configuration guarantees the maximum achievable RCS thus providing a global lower BER of the chipless RFID communication system. We believe that the BER is the more meaningful and fair figure of merit for comparing the performance of different tags than bit/cm2 or bit/Hz since the increase of encoded information of the tag is useful only if it can be correctly decoded.

Synchronism and Direction Detection in High-Resolution/High-Density Electromagnetic Encoders

Recently, electromagnetic encoders with synchronous reading and direction detection capability have been reported. Such structures are useful for the implementation of (i) displacement/velocity sensors and (ii) chipless-RFID systems based on near-field coupling and sequential bit reading. In the latter, synchronous reading and motion direction detection are a need in order to avoid false readings of the identification (ID) code, if the relative velocity between the reader and the encoder is not constant, and to read the correct ID code (rather than the inverse one), respectively. On the other hand, synchronous reading and motion direction detection, are essential to determine the encoder direction in displacement/velocity sensors, as well as to provide the absolute position of the encoder, provided that the whole encoder is encoded with the Bruijn sequence. In this paper, synchronous reading and direction detection in high-resolution/high-density electromagnetic encoders based on chains of linearly-shaped metallic inclusions are reported. To this end, it is necessary to add two chains of metallic inclusions to the one containing the ID code. In the reader side, three harmonic signals are necessary in order to generate the clock signals and to obtain the ID code. The reader consists of a microstrip line loaded with three pairs of open-ended folded stubs positioned face-to-face by their extremes. By displacing the encoder chains over the extreme of the stubs, at short distance, stub coupling is enhanced when a metallic inclusion lies on top of the stubs, and the frequency response of the reader is shifted towards lower frequencies. Thus, by injecting three (properly tuned) harmonic signals at the input port of the microstrip transmission line, three amplitude modulated (AM) signals are generated by tag motion at the output port of such transmission line, and the envelope functions contain the velocity, the ID code and the absolute position. The reported reader/encoder system exhibits superior space resolution and information density as compared to other similar systems based on synchronous reading.

Development of Cross-Polar Orientation-Insensitive Chipless RFID Tags

Chipless RFID is a potential wireless identification technology for tracking and identification applications. However, there are still some unsolved challenges in the implementation of this system in real-world applications. One of the crucial challenges in chipless RFID systems is the effect of the dynamic environment and the effect of the attached item on the tag response, which causes interference in the tag ID measurement process. One approach to solve this problem at the tag design level is to use cross-polar chipless RFID tags, which conventionally are orientation-sensitive. However, even though it is desirable to detect the tag ID in different orientations (in 2-D at least) for practical applications, having both cross-polar RCS and orientation insensitivity in a tag is a challenge. Therefore, in this article, a technique to develop cross-polar resonators and unit cells that display a consistent reflection regardless of tag orientations is proposed. The results of the simulation and measurement of a few samples of the proposed tags are depicted to demonstrate the feasibility of using the proposed tags in a system with reflective items in the background. Five cross-polar structures with low sensitivity to orientation are introduced in this article. Among all structures, the flower-shaped unit cell provides a very consistent cross-polar response at different rotational angles with a maximum of 11 dB peak RCS variation.

Compact multi‐band chipless RFID resonators for identification and authentication applications

Moving towards commercialisation of chipless RFID systems, there are still some pressing issues. In the tag design level, the issues are high data capacity in the order of tens of bits, compact design and the immunity to the interference which can be achieved by a cross-polar response. It is demanded to yield the highest numbers of bits in a tag ID while the occupied physical space and allocated bandwidth are minimised. Increasing the number of resonators in a conventional multi-resonator chipless tag causes a significant tag size growth. The destructive coupling effect between resonators in multi-resonator tags is another critical issue. In this Letter, to combat those issues, a multi-band resonator is proposed. The comb-shaped tags contain both co-polar and cross-polar Radar cross section (RCS) responses, hence they can be used in high reflective environments. Two samples of 14-bit and 40-bit comb-shaped tags are designed, which have physical sizes of 17 mm × 13 and 30 mm × 45 mm, respectively. The promising performance of the proposed structure, which is beyond the state of art hybrid and frequency-domain tags, shows the potential of the proposed designs for identification and authentication applications.

Practical Performance Comparison of 1-D and 2-D Decoding Methods for a Chipless RFID System in a Real Environment

A comparison of four decoding methods for a Chipless RFID system was executed by analyzing the read success rate of a frequency-position coding tag with 12-bit of capacity. Scattering parameter measurements have been performed in a real environment, using a fixed antenna, with the tag moving within a $30\times 30\times 30$ cm3 volume. Optimal parameters have been obtained based on the decoding success rates of the practical measurements, for background calibration, time-gating, Short Time Fourier Transform (STFT) and Short Time Prony Analysis (STPA) methods. STFT and time-gating showed the best volume reading performance. Time-gating has been found to be the best decoding method for the chipless RFID system employed, as it presented the best performance amongst the other methods while being a less complex, 1-D, approach.

A high capacity tunable retransmission type frequency coded chipless radio frequency identification system

This article presents a 12-bit frequency coded chipless RFID system in the frequency range of 3 to 6 GHz. The system consists of a fully printable chipless tag and a pair of high-gain reader antennas. The tag also incorporates its own antennas to improve the read range. Information is encoded into frequency spectrum using a multi-resonant circuit. The circuit consists of multiple microstrip U and L-shaped open stub resonators patterned in a unique configuration. The proposed configuration aids in capturing more data in a reduced space as well as tunable frequency operation. Tag and reader antennas utilize techniques such as stepped impedance feeding line, defective partial ground plane, and stair-step patch structure to achieve wide-band impedance bandwidth in miniature size. The results of the wireless measurements in the non-anechoic environment show that the proposed system has a reading range of more than 20 cm. The presented system possesses great potential for low-cost short-range inventory tracking.

Enhancing the Per-Unit-Length Data Density in Near-Field Chipless-RFID Systems With Sequential Bit Reading

In this letter, a novel near-field chipless-Radio Frequency Identification (RFID) system with sequential bit reading is reported. The main novelty concerns the tags, implemented by means of straight half-wavelength resonators separated by a small distance and forming a linear chain in the direction orthogonal to their axis. By this means, the number of resonant elements (or bits) per unit length of the tag is significantly increased, as compared to the previous implementations, with the result of very competitive tags in terms of data density. The active part of the reader is a microstrip line loaded with a shunt stub. Through a proper design, it is possible to achieve a significant excursion of the transmission coefficient when the stub is either loaded or unloaded with a functional resonant element of the tag. Consequently, by tag motion over the reader (stub), it is possible to infer the Identification (ID) code by simply injecting a harmonic signal to the input port of the stub-loaded line (the ID code is contained in the amplitude modulated, signal at the output port). This system has been validated by considering 100 b tags exhibiting a surface data density of 4.9 b/cm2. However, the relevant parameter is the data density per unit length, as high as 16.7 b/cm, i.e., much larger than previous values reported in the literature in similar chipless-RFID systems.

Very Low-Cost 80-Bit Chipless-RFID Tags Inkjet Printed on Ordinary Paper

This paper presents a time-domain, chipless-RFID system with 80-bit tags inkjet-printed on ordinary DIN A4 paper. The tags, consisting of a linear chain of resonant elements (with as many resonators as the number of identification bits plus header bits), are read sequentially and by proximity (through near-field coupling). To this end, a transmission line, fed by a harmonic (interrogation) signal tuned to the resonance frequency of the tag resonators (or close to it), is used as a reader. Thus, during reader operation, the tag chain is mechanically shifted over the transmission line so that the coupling between the line and the functional resonant elements of the tag chain is favored. Logic states that ‘1’ and ‘0’ are determined by the functionality and non-functionality (resonator detuning), respectively, of the resonant elements of the chain. Through near-field coupling, the transmission coefficient of the line is modulated and, as a result, the output signal is modulated in amplitude (AM), which is the identification code contained in the envelope function. As long as the tags are inkjet-printed on ordinary DIN A4 paper, the cost is minimal. Moreover, such tags can be easily programmed and erased, so that identical tags can be fabricated on a large scale (and programmed at a later stage), further reducing the cost of manufacture. The reported prototype tags, with 80 bits of information plus four header bits, demonstrate the potential of this approach, which is of particular interest to secure paper applications.

Near-Field Chipless Radio-Frequency Identification (RFID) Sensing and Identification System with Switching Reading.

A chipless radio-frequency identification (chipless-RFID) and sensing system, where tags are read by proximity (near-field) through a switch, is presented. The tags consist of a set of identical resonant elements (split-ring resonators or SRRs), printed or etched at predefined and equidistant positions, forming a linear chain, each SRR providing a bit of information. The logic state (‘1’ or ‘0’) associated with each resonator depends on whether it is present or not in the predefined position. The reader is an array of power splitters used to feed a set of SRR-loaded transmission lines (in equal number to the number of resonant elements, or bits, of the tag). The feeding (interrogation) signal is a harmonic (single-tone) signal tuned to a frequency in the vicinity of the fundamental resonance of the SRRs. The set of SRR-loaded lines must be designed so that the corresponding SRRs are in perfect alignment with the SRRs of the tag, provided the tag is positioned on top of the reader. Thus, in a reading operation, as long as the tag is very close to the reader, the SRRs of the tag modify (decrease) the transmission coefficient of the corresponding reader line (through electromagnetic coupling between both SRRs), and the amplitude of the output signal is severely reduced. Therefore, the identification (ID) code of the tag is contained in the amplitudes of the output signals of the SRR-loaded lines, which can be inferred sequentially by means of a switching system. Unlike previous chipless-RFID systems based on near-field and sequential bit reading, the tags in the proposed system can be merely positioned on top of the reader, conveniently aligned, without the need to mechanically place them across the reader. Since tag reading is only possible if the tag is very close to the reader, this system can be also used as a proximity sensor with applications such as target identification. The proposed chipless-RFID and sensing approach is validated by reading a designed 4-bit tag. For identification purposes, this system is of special interest in applications where a low number of bits suffice, and tag reading by proximity is acceptable (or even convenient). Applications mostly related to secure paper, particularly involving a limited number of items (e.g., exams, ballots, etc.), in order to provide authenticity and avoid counterfeiting, are envisaged. As a proximity sensor, the system may be of use in detecting and distinguishing different targets in applications such as smart packaging.

Near-Field Chipless-RFID System With Erasable/Programmable 40-bit Tags Inkjet Printed on Paper Substrates

In this letter, a chipless radio frequency identification (chipless-RFID) system with erasable/programmable 40-bit tags inkjet printed on paper substrates, where tag reading proceeds sequentially through near-field coupling, is presented for the first time. The tags consist of a linear chain of identical split ring resonators (SRRs) printed at predefined and equidistant positions on a paper substrate, and each resonant element provides a bit of information. Tag programming is achieved by cutting certain resonant elements, providing the logic state “0” to the corresponding bit. Conversely, tags can be erased (all bits set to “1”) by short circuiting those previously cut resonant elements through inkjet. An important feature of the proposed system is the fact that tag reading is possible either with the SRR chain faced up or faced down (with regard to the reader). To this end, two pairs of header bits (resonators), with different sequences, have been added at the beginning and at the end of the tag identification chain. Moreover, tag data storage capacity (number of bits) is only limited by the space occupied by the linear chain. The implementation of tags on paper substrates demonstrates the potential of the proposed chipless-RFID system in secure paper applications, where the necessary proximity between the reader and the tag, inherent to near-field reading, is not an issue.

Application of metamaterial concepts to sensors and chipless RFID

Several strategies for the implementation of microwave sensors based on the use of metamaterial-inspired resonators are pointed out, and examples of applications, including sensors for dielectric characterization and sensors for the measurement of spatial variables, are provided. It will be also shown that novel microwave encoders for chipless RFID systems with very high data capacity can be implemented. The fields of applications of the devices discussed in this talk include dielectric characterization of solids and liquids, angular velocity sensors for space applications, and near-field chipless RFID systems for secure paper applications, among others.

Document Verification: A Cloud-Based Computing Pattern Recognition Approach to Chipless RFID

In this paper, we propose a novel means of verifying document originality using chipless RFID systems. The document sender prints a chipless RFID tag into the paper and does a frequency scanning in the 57–64 GHz spectrum of the document. The results of scattering parameters in individual step frequencies are stored in a cloud database, denoised and passed to pattern classifiers, such as support vector machines or ensemble networks. These supervised learners train themselves based on these data on the remote/cloud computer. The document receiver verifies this frequency fingerprint by using the same scanning method, sending the scattering parameters to the cloud server and getting the decoded data. Paper originality is verified if the decoded data are as expected. The advantages of our cloud chipless RFID processing deployments are cost reduction and increased security and scalability.

Near-Field Chipless-RFID System With High Data Capacity for Security and Authentication Applications

A high data capacity chipless radio frequency identification (chipless-RFID) system, useful for security and authentication applications, is presented in this paper. Reading is based on the near-field coupling between the tag, a chain of identical split-ring resonators (SRRs) printed on a (typically flexible) dielectric substrate (e.g., liquid crystal polymer, plastic, and paper), and the reader. Encoding is achieved by the presence or absence of SRRs at predefined (equidistant) positions in the chain, and tag identification (ID) is based on sequential bit reading. Namely, the tag must be longitudinally displaced, at short distance, over the reader, a microstrip line loaded with an SRR and fed by a harmonic signal. By this means, the harmonic signal is amplitude modulated, and the (ID) code is contained in the envelope function, which can be obtained by means of an envelope detector. With this system, tag reading requires proximity with the reader, but this is not an issue in many applications within the domain of security and authentication (e.g., secure paper for corporate documents and certificates). Several circularly shaped 40-bit encoders (implemented in a commercial microwave substrate), and the corresponding reader, are designed and fabricated as proof-of-concept demonstrators. Strategies for programming the tags and a first proof-of-concept chipless-RFID tag fabricated on plastic substrate through inkjet printing are included in this paper.